1. Field of the Invention
This invention relates to an improved process for the manufacture of potassium polyphosphates from potassium raw materials, and more particularly to a method for the production of potassium polyphosphates from potassium fluosilicates and mineral acids and the resulting novel products.
2. Description of the Prior Art
A fertilizer may be defined as a compound or mixture of compounds which contain nitrogen and/or phosphorus and/or potassium in such forms that they are available for assimilation by plants. The plant food content of the fertilizer is often expressed as the total of the percentages of nitrogen (N), phosphate (P.sub.2 O.sub.5), and potassium (K.sub.2 O). Many different types of fertilizers have been prepared and are used commercially including various ammonium phosphates, potassium phosphates and the like.
One class of materials which has been found useful as fertilizers when produced from potassium, and as useful in the detergent art when prepared from sodium, are the so-called crystalline condensed phosphates. Materials of this type have been prepared heretofore by heating of various raw materials at high temperatures or calcined. For example, in the preparation of tetrasodium pyrophosphates, anhydrous disodium orthophosphate may be calcined at any temperatures between 300-900.degree. C. Condensed potassium phosphates have been produced as described for example in U.S. Pat. No. 3,600,152.
None of these prior art procedures however, has been concerned with a product having fertilizer characteristics which can be prepared from a material which has heretofore been considered as a by-product. Thus, in the formation of wet process phosphoric acid by the acidulation of phosphate rock with sulfuric acid, there is often produced a by-product from the reaction of the fluorine and silicon contained in the phosphate rock with any alkali metal present, the product formed being alkali metal fluosilicate. This material may be subsequently recovered from either precipitated gypsum solids or precipitated from the wet process phosphoric acid.
The alkali metal fluosilicate has the formula M.sub.2 SiF.sub.6, where M is alkali metal, usually potassium or sodium. As pointed out above, this material has previously been treated as a by-product as there was no commercially available procedure for its conversion to a useful product. With the limitations now being placed on phosphoric acid plants to prevent evolution of the fluorides during the acidulation, it is to be expected that more and more alkali metal silicofluorides will be produced and will either have to be converted to a useful form or disposed of in other ways. This would, of course, be an uneconomic aspect of any phosphoric acid plant.
It is known from U.S. Pat. No. 3,689,216 to Brown that potassium fluosilicate may be converted to gaseous hydrogen fluoride by reacting the fluosilicate with excess sulfuric acid at temperatures of 20-100.degree. C. so as to produce gaseous silicon tetrafluoride and a fluoride-containing by-product and heating the by-product to a temperature of 80-300.degree. C. to produce the hydrogen fluoride product. It is also known from U.S. Pat. No. 2,874,027 to convert mixtures of alkali metal fluorides in phosphoric acid into glassy alkali metal polymetaphosphate by reaction at high temperatures. A similar product is prepared in U.S. Pat. No. 3,049,419 as well as U.S. Pat. No. 3,361,523.
In none of these prior art patents or of any other prior art which Applicant is aware, are there disclosures by which alkali metal fluosilicate can be converted to useful forms which have valuable fertilizer characteristics.